In recent years, semiconductor devices have become more integrated, and structures of semiconductor elements have become more complicated. Further, a number of layers in multilayer interconnections used for a logical system has been increased. Accordingly, irregularities on a surface of a semiconductor device become increased, so that step heights on the surface of the semiconductor device tend to be larger. This is because, in a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, then micromachining processes, such as patterning or forming holes, are performed on the semiconductor device, and these processes are repeated many times to form subsequent thin films on the semiconductor device.
When a number of irregularities is increased on a surface of a semiconductor device, the following problems arise. A thickness of a film formed in a portion having a step is relatively small when a thin film is formed on a semiconductor device. An open circuit is caused by disconnection of interconnections, or a short circuit is caused by insufficient insulation between interconnection layers. As a result, good products cannot be obtained, and a yield tends to be reduced. Further, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after long-term use. At a time of exposure in a lithography process, if an irradiation surface has irregularities, then a lens unit in an exposure system is locally unfocused. Therefore, if the irregularities of the surface of the semiconductor device are increased, then it becomes problematic in that it is difficult to form a fine pattern itself on the semiconductor device.
Accordingly, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. A most important one of planarizing technologies is CMP (Chemical Mechanical Polishing). In chemical mechanical polishing, with use of a polishing apparatus, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus comprises a polishing table having a polishing surface constituted by a polishing pad, and a substrate holding apparatus, which is called a top ring or a carrier head, for holding a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing table under a predetermined pressure by the substrate holding apparatus. At this time, the polishing table and the substrate holding apparatus are moved relatively to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that a surface of the semiconductor wafer is polished to a flat mirror finish.
In such a polishing apparatus, if a relative pressing force between the semiconductor wafer being polished and the polishing surface of the polishing pad is not uniform over an entire surface of the semiconductor wafer, then the semiconductor wafer may insufficiently be polished or may excessively be polished at some portions depending on a pressing force applied to those portions of the semiconductor wafer. Therefore, it has been attempted to form a surface, for holding a semiconductor wafer, of a substrate holding apparatus by an elastic membrane made of an elastic material such as rubber, and to supply fluid pressure such as air pressure to a backside surface of the elastic membrane to uniformize pressing forces applied to the semiconductor wafer over an entire surface of the semiconductor wafer.
Further, the polishing pad is so elastic that pressing forces applied to a peripheral portion of the semiconductor wafer being polished become non-uniform, and hence only the peripheral portion of the semiconductor wafer may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, there has been used a substrate holding apparatus in which a semiconductor wafer is held at its peripheral portion by a guide ring or a retainer ring, and an annular portion of the polishing surface that corresponds to the peripheral portion of the semiconductor wafer is pressed by the guide ring or retainer ring.
A conventional substrate holding apparatus will be described below with reference to FIGS. 29A and 29B. FIGS. 29A and 29B are fragmentary cross-sectional views showing a conventional substrate holding apparatus.
As shown in FIG. 29A, the substrate holding apparatus has a top ring body 2, a chucking plate 6 housed in the top ring body 2, and an elastic membrane 80 attached to the chucking plate 6. The elastic membrane 80 is disposed on an outer circumferential portion of the chucking plate 6, and is brought into contact with a circumferential edge of a semiconductor wafer W. An annular retainer ring 3 is fixed to a lower end of the top ring body 2, and presses a polishing surface near the outer circumferential edge of the semiconductor wafer W.
The chucking plate 6 is mounted on the top ring body 2 through an elastic pressurizing sheet 13. The chucking plate 6 and the elastic membrane 80 are vertically moved in a certain range with respect to the top ring body 2 and the retainer ring 3 by fluid pressure. The substrate holding apparatus having such a structure is referred to as a so-called floating-type substrate holding apparatus. A pressure chamber 130 is defined by the elastic membrane 80, a lower surface of the chucking plate 6, and an upper surface of the semiconductor wafer W. A pressurized fluid is supplied into the pressure chamber 130, thereby lifting the chucking plate 6 and simultaneously pressing the semiconductor wafer W against a polishing surface. In this state, a polishing liquid is supplied onto the polishing surface, and a top ring (the substrate holding apparatus) and the polishing surface are rotated independently of each other, thus polishing a lower surface of the semiconductor wafer W to a flat finish.
After this polishing process is finished, the semiconductor wafer W is attracted under vacuum and held by the top ring. The top ring is moved to a transfer position while holding the semiconductor wafer W, and then a fluid (e.g., a pressurized fluid or a mixture of nitrogen and pure water) is ejected from a lower portion of the chucking plate 6 so as to release the semiconductor wafer W.
However, in the conventional floating-type substrate holding apparatus described above, when the chucking plate 6 is moved upwardly for pressing the semiconductor wafer W, the elastic membrane 80, which is held in contact with an outer circumferential edge of the semiconductor wafer W, is lifted by the chucking plate 6, thus causing an outer circumferential edge of the elastic membrane 80 to be brought out of contact with the semiconductor wafer W. Consequently, a pressing force applied to the semiconductor wafer W is locally changed at the outer circumferential edge of the semiconductor wafer W. As a result, a polishing rate is lowered at the outer circumferential edge of the semiconductor wafer W and is increased at a region located radially inwardly of the outer circumferential edge of the semiconductor wafer W.
As a hardness of the elastic membrane becomes higher, such a problem becomes worse. Therefore, it has been attempted to use an elastic membrane having a low hardness so that a contact area between the elastic membrane and the semiconductor wafer is kept constant. However, in the floating-type substrate holding apparatus, the semiconductor wafer W is polished while the retainer ring 3 is held in sliding contact with the polishing surface. Accordingly, the retainer ring 3 tends to wear with time, resulting in a reduction in a distance between the semiconductor wafer W and the chucking plate 6 (see FIG. 29B). Consequently, a pressing force applied to the outer circumferential edge of the semiconductor wafer W is changed, and hence the polishing rate is changed at the outer circumferential edge of the semiconductor wafer W, thus causing a change in a polishing profile. Further, because of such a drawback, it is necessary to replace a worn retainer ring at an early stage, and hence a lifetime of the retainer ring is limited to a short period.
In addition to the above problem, the conventional substrate holding apparatus has another problem as follows: When a polishing process is to be started, pressurized fluid is supplied to the pressure chamber while the elastic membrane and the semiconductor wafer may not be sufficiently held in close contact with each other. As a result, the pressurized fluid is liable to leak from a gap between the elastic membrane and the semiconductor wafer.
Further, in a process of releasing the semiconductor wafer from the top ring, the following problem arises: If a film of nitride or the like is formed on a backside surface (upper surface) of the semiconductor wafer, then the elastic membrane and the semiconductor wafer adhere to each other. Therefore, when releasing the semiconductor wafer, the elastic membrane may not be brought out of contact with the semiconductor wafer. In this state, if a pressurized fluid is continuously ejected to the semiconductor wafer, the elastic membrane is stretched while keeping contact with the semiconductor wafer. As a result, the semiconductor wafer is deformed, or broken at worst, due to a fluid pressure.
Furthermore, still another problem arises in the conventional substrate holding apparatus as follows: The pressure chamber constituted by the elastic membrane is deformed due to a fluid pressure. Therefore, the elastic membrane is locally brought out of contact with the semiconductor wafer as the pressurized fluid is supplied to the pressure chamber. Consequently, a pressing force applied to the semiconductor wafer is locally lowered, and hence a uniform polishing rate cannot be obtained over an entire polished surface of the semiconductor wafer.
As a hardness of the elastic membrane becomes higher, such a problem becomes worse. Therefore, as already described, it has been attempted to use an elastic membrane having a low hardness so that a contact area between the elastic membrane and the semiconductor wafer is kept constant. However, because the elastic membrane having a low hardness has a low mechanical strength, the elastic membrane tends to suffer cracking, and is thus required to be replaced frequently.